1. Field of the Invention
This invention relates generally to circuit interrupters and, more particularly, to circuit interrupters including an arc fault trip mechanism which responds to sputtering arc faults. The invention also relates to methods for providing series arc protection for electrical circuits.
2. Background Information
Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of electrodes. An arc fault is an unintentional arcing condition in an electrical circuit. Arc faults can be caused, for instance, by worn insulation between adjacent bared conductors, by exposed ends between broken conductors, by faulty electrical connections, and in other situations where conducting elements are in close proximity.
Arc faults in systems can be intermittent since the magnetic repulsion forces generated by the arc current force the conductors apart to extinguish the arc. Mechanical forces then bring the conductors together again in order that another arc is struck.
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
Recently, there has been considerable interest in providing protection against arc faults. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average RMS current to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads, which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. Nos. 5,224,006; and 5,691,869.
For example, an arc fault circuit interrupter (AFCI) is a device intended to mitigate the effects of arc faults by functioning to deenergize an electrical circuit when an arc fault is detected. Non-limiting examples of AFCIs include: (1) arc fault circuit breakers; (2) branch/feeder arc fault circuit interrupters, which are intended to be installed at the origin of a branch circuit or feeder, such as a panelboard, and which may provide protection from ground faults (e.g., greater than 40 mA) and line-to-neutral faults (e.g., greater than 75 A); (3) outlet circuit arc fault circuit interrupters, which are intended to be installed at a branch circuit outlet, such as an outlet box, in order to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing, and which may provide protection from line-to-ground faults (e.g., greater than 75 A) and line-to-neutral faults (e.g., 5 to 30 A, and greater than 75 A); (4) cord arc fault circuit interrupters, which are intended to be connected to a receptacle outlet, in order to provide protection to an integral or separate power supply cord; (5) combination arc fault circuit interrupters, which function as either a branch/feeder or an outlet circuit AFCI; and (6) portable arc fault circuit interrupters, which are intended to be connected to a receptacle outlet and provided with one or more outlets.
UL 1699 is a specification that governs the performance of AFCI products including branch/feeder type (AVZQ); outlet circuit type (AWCG); portable type (AWDO); cord type (AWAY); and combination type (AWAH) AFCIs. A carbonized path arc clearing time test is conducted in which the total clearing time before the AFCI trips shall not exceed specified arc test clearing times based upon different levels of test current (i.e., 5 A; 10 A; 15 A or 20 A; 22.5 A or 30 A). UL 1699 requires that the combination type AFCI must detect and interrupt the parallel combination of compressor and arc within a one-second clearing time for an arc test current of 5 A (resistive load).
UL 1699 specifies detection of series arcs only when loads on an associated electric distribution system are in steady-state operation.
The step application of a load voltage often results in significant load current transients. When a load is energized in an electrical power distribution system, there can be an initial transient current that decays into a periodic, stable current when the load reaches steady-state operation. In many cases, this is due to energy storage in the load, such as capacitive elements (which store energy in electric fields) or inductive elements (which store energy in magnetic fields). When a forcing function (e.g., a 60 Hz, 120 VRMS voltage source) is applied to a load, the load current exhibits a “natural” response which decays with the time constant(s) of the load, and a “forced” or “steady-state” response. The initial burst of increased current when the load is first energized performs the function of supplying stored energy required for the load to operate normally. For example, the initial transient of a computer load with a rectifier/capacitor input characteristic is due to capacitive energy storage, the initial transient of an electric motor is due to inductive energy storage, and the initial transient of a vacuum sweeper is due to inductive energy storage and the load's initial inertial energy storage.
Initial current transients may also result from load characteristics other than energy storage. One example of this is the impedance of an incandescent light, which varies greatly over the normal range of operating temperatures. When voltage is applied to incandescent lights at room temperature, the light-producing elements rapidly heat up to a steady-state temperature, which is accompanied by a significant increase in impedance and a consequent drop in current. An example of step voltage application 1A and current response 1B in incandescent lighting controlled by a dimmer (not shown) is shown in FIG. 1.
In contrast to the step application of load voltage, series arcs cause relatively minor variations in load voltage and current. For example, for a 60 Hz, 120 VRMS voltage source, when an arc occurs in series with a load, the sustained arc voltage can be about 20 to about 40 VPEAK, depending on the conditions that create the arc. The arc voltage is subtracted from the source voltage, so that when an arc occurs, the voltage excitation at the load declines by about 10% to about 20%. The onset of series arcing is not accompanied by a load current transient like the type that accompanies a step application of voltage. Compared to current transients associated with applying a step voltage to a load, there is no dramatic change in the amplitude of the load currents. FIG. 2 shows series arc voltage 1C, load voltage 1D and load current 1E versus time for the onset of arcing in series with dimmer-controlled incandescent lighting (not shown) in which the arc is produced by using a carbon-copper arc generator (e.g., as specified by UL 1699, § 58.1.3) in series with the load.
As a result, step voltages (such as, for example, applying a voltage source to an unenergized load) can cause major variations in load current, while series arcs tend to cause only relatively minor variations in load current. Hence, any series arc detection algorithm must ignore major variations in load current and trip only on relatively minor variations in load current. It is believed that this conclusion flies in the face of conventional arc fault detection practice, particularly for parallel arc detection algorithms in which minor variations in load current are ignored and major variations in load current are considered the arc signatures of primary interest.
U.S. patent application Ser. No. 10/895,158 discloses that various arc fault algorithms clear a trip tally value whenever the load current exceeds a predetermined limit (e.g., 45 A; 30 ARMS) for series arc protection (e.g., series arc protection is defined by UL 1699).
There is, therefore, room for improvement in arc fault circuit interrupters and methods for providing series arc protection.